I believe that Bob V. has some sort of spintech side exit system on his clone. I could be wrong, but his site is
www.gt350clone.com
I read up on the spintech systems, and from what I hear, they sound pretty good. I heard that they will act fairly mild at lower rpms, but when you get on it, they'll REALLY howl.
So with the side exit system, if this is just a weekend car, go for it, but if you're going to be making daily runs in the car, I would think twice. Not only would more fumes be going into the car, but the noise would be pretty incredible.
As far as them droning, I'm not sure, but they are chambered, so I'm sure they would drone. Probably not as much as the flows though.
I know this is all opinionated, but no one else responded, so I thought i'd give you atleast something...